1. Field of the Invention
The present invention relates to improved saturated polyesters comprising ethylene glycol terephthalate recurring units, and to containers shaped therefrom useful for the packaging of foodstuffs and beverages. The invention also relates to a process for the preparation of such improved polyesters.
2. Description of the Prior Art
It is well known to this art that thermoplastic polyesters comprising ethylene glycol terephthalate regularly recurring units constitute valuable starting materials for the packaging of food products, in the form of films, sheets and containers, and especially in the form of bottles intended for storing quiescent or still liquids or carbonated beverages.
One of the essential properties which such packaging, and more particularly a bottle, must possess, is transparency and brightness. By drawing from the amorphous state under conditions of biaxial orientation, poly(ethylene glycol)terephthalate retains good transparency while at the same time displays the advantage of an oriented crystalline phase which considerably enhances its mechanical properties and permits a reduction in the permeability to gases. However, the transparency and the brightness can be adversely affected by the formation of crystalline spherulites, especially in the case of containers with relatively thick walls. The production of bottles by injection molding and/or extrusion-blowing, or blow molding, necessarily involves the prior production of a preform having thick walls, which is obtained by quenching from the molten state, and the formation of crystalline spherulites at the preform stage adversely affects the transparency of the finished article. It has thus proved advantageous to recommend grades of polyester having a slow rate of crystallization.
Another property which packaging material fabricated from a polymeric substance must possess is the absence of any compound or additive which is capable of migrating into the food product or the drink and of impairing its taste or its smell. It too is known, though, that polyester resins release acetaldehyde by degradation, and this possesses a strong smell and a characteristic taste which appear at extremely low concentrations.
The aforesaid problem is particularly significant and acute in the packaging (bottling) of mineral water. Thus, polyester bottles intended for aerated mineral water must have an acetaldehyde concentration of less than 5 ppm (parts per million of polyester by weight/weight) in the wall members thereof, for a volume of between 1 liter and 2 liters, if it is desired to ensure that, under storage conditions, at temperatures which can reach 45.degree. C., the amount of acetaldehyde which is capable of migrating into the liquid remains less than the detectable organoleptic threshold, that is to say 40-50 ppb (parts per billion by weight/weight of liquid). The amount of acetaldehyde present in the finished shaped articles depends both on the residual amount present in the polyester granules, before they are converted, and on the amount formed during the conversion in the molten state, as a result of degradation under the conditions of shear and temperature which are required for molding.
Various processes have to date been proposed for reducing the concentration and/or the reformation of acetaldehyde in poly(ethylene glycol)terephthalate resins. Typically, such processes feature operating conditions, during the stages of drying, crystallization or post-condensation of the precursor low molecular weight prepolymer, which are of the kind commonly used in the manufacture of high-viscosity resins.
Thus, according to published French Patent Application No. 78/23,635, for example, the drying of the polyester chips is carried out at 180.degree.-230.degree. C. for 4 to 12 hours, in an inert atmosphere, in order to reduce the acetaldehyde concentration in the chips to less than 2.5 ppm. According to published French Patent Application No. 79/10,061, a reduction in the rate of formation of acetaldehyde is achieved by treating the completely polymerized polyester in molten state, under an inert atmosphere.
And according to published French Patent Application No. 79/11,981, the polymer is crystallized by heating to 180.degree.-220.degree. C., and the post-condensation is carried out at a temperature above the crystallization temperature.
U.S. Pat. No. 4,154,920 features a two-step process starting from a prepolymer having an intrinsic viscosity of 0.1 to 0.3 dl/g, in accordance with which the prepolymer is first condensed in the molten state to form a thin layer and then post-condensed in the solid state.
Finally, according to published French patent application No. 79/12,337, the polyester pellets are post-condensed and stabilized by heating at 180.degree.-220.degree. C. for 2 to 20 hours, in air, thus increasing the intrinsic viscosity to a value of between 0.60 and 0.97 dl/g.